Wednesday, August 12, 2009

Measuring Past Climates

Climate is a collection of all the long-term averages of the daily weather variables, such as rainfall, wind speed and direction, humidity, hours of sunlight and temperature. It is usually considered that a minimum data-base of thirty years is required to provide a useful description of the climate of a particular area.

In Australia weather information has been collected by various sources for about 150 years, and this information is now proving valuable in comparing our climate today with that of previous eras.

In Europe and America formal weather records date back much further – typically back to around 300 years ago, with the British Navy in particular accumulating weather information from localities all around the globe.

Records from other sources such as church registers, family histories and even art and literature also provide some useful information on past climate trends. But in reality these sources do little more than scratch the surface of climate investigation.

The Earth is estimated to be around 4.6 billion years old, so 300 years of recorded climate history is really only a drop in the ocean. Instead we must go to other sources to look for evidence of climate patterns and fortunately there are many. Climatologists look for what is called proxy data - evidence left behind in the natural world that is either the direct or indirect result of the climate at the time. This has proved to be a fascinating and revealing area of climate research.

Fossil evidence
The interpretation of fossil evidence enables us to look through a keyhole and catch intriguing glimpses of long ago climates that were experienced across our planet.

Many fossils are millions of years old and provide valuable insights into climates that were experienced in the times of the dinosaur and even well before. Ferns, for instance, require warm and humid conditions to survive and the discovery of fossilised specimens provides evidence of a past climate of this nature. Dating procedures can then put some sort of time frame on this. Changes in vegetation patterns over time that are also revealed through fossil evidence can be valuable indicators of climate change.

Petrified pine cone from the Jurassic Era ~210 million years ago collected from Patagonia.(Image from Wikipedia Commons - click to enlarge)

The fossilised evidence of the presence of large grazing animals also infers the likelihood of an abundant cover of vegetation and again points to a warm and wet climate at the time.

Ice Cores
Ice caps in the Antarctic and Greenland are hundreds of metres thick in some areas, and are the result of snow falling over the millennia and gradually compacting into huge ice sheets. The ice that lies down towards the base of these sheets in Antarctica is over half a million years old and minute bubbles captured inside contain samples of the ancient atmosphere. Scientists drill deep down into the sheets and retrieve long ice cores that contain a great deal of information about past climates.Ice cores taken at the Russian Antarctic base at Vostok(Image from Wikipedia Commons - click to enlarge)

Analysis of the trapped air bubbles enables scientists to reconstruct the gas concentrations contained in the atmosphere of the time and compare this with modern day figures. The thickness of the annual ice layers also reveal in which years there were heavy snowfalls and trapped dust particles point to eras in which there was increased storminess and volcanic activity.

Glaciers
These large rivers of ice travel at very slow speed down mountain sides under the influence of gravity and are capable of gouging out huge “U” shaped valleys as they do so.

Geologists have learned to identify these valleys and the associated rock debris trail that is produced along each side of the ice flow and can therefore identify where glaciers have been in the past.

There are many areas in Europe, particularly through parts of France and Switzerland where valleys of this type are found and this indicates that there have been much colder epochs in earlier times than now. Glacial valleys that contain no ice are therefore evidence of climate change, and processes that can estimate the age of these give us some idea of when these cold periods occurred. There are some good examples of old glacial valleys across the central plateau area of Tasmania.

The Aletsch Glacier in Switzerland(Image from Wikipedia Commons - click to enlarge)

In 1840 the Swiss scientist Louis Agassiz was the first to suggest that there had been “ice ages” in the past, and although initially disbelieved by the scientists of the day, it is now recognised as proven theory.

Stalactites and Stalagmites

These amazing icicle like rock formations that grow in caves have fascinated humans for centuries and in more recent times have also found to be valuable climate indicators for the local area.

Stalactites extend down from the ceilings of caves, whereas stalagmites grow upwards from the cave floor. They are produced by dripping water rich in calcium carbonate. This forms deposits that gradually grow and harden over time. Many formations around the world are at least 100,000 years old, with some even far older.

But the way they grow carries with it a great deal of information about the past climate in the local area as geologists are able to deduce periods of rapid or slow growth by examining the small-scale structure of the formations. And this rate of growth depends on the rainfall with wet periods producing more dripping water within the cave and higher growth rates.

Scientists are then able to determine the age of the formations using standard dating techniques and deduce a rainfall timeline. In addition to rainfall patterns, stalactites and stalagmites also contain information about past temperature trends across the area.

Oxygen atoms are bound up within water, and these come in two forms, known as “heavy” and “light”. The ratio of these is temperature sensitive, and this data can be retrieved from stalactites and stalagmites allowing a temperature timeline to be constructed.

Corals

Coral formations that are common in tropical oceans around the world are quite ancient, typically between 5 and 10 thousand years old, and because they are highly sensitive to the state of the environment, they have proved invaluable in reconstructing past climates in tropical areas.

Corals react strongly to three main variables. These are the temperature, salinity and acidity of the surrounding seawater and these factors are all the end result of the climate of the area.

During periods of high rainfall, ocean waters near coastlines become minutely diluted by the excess of fresh rainwater and become less saline or “salty”. This produces a slightly different growth pattern in the coral than at other times. Scientists have learned to detect and date this difference and deduce past rainfall patterns across the area.Pillar Coral located at The Florida Keys National Marine Sanctuary(Image from Wikipedia Commons - click to enlarge)

The ideal water temperature in which coral thrives is around 27C but it is remarkably sensitive to any long-term variations from this, even by only a few degrees. Changes in the structure of coral growth patterns during periods of temperature change have enabled climatologists to build up long-term temperature profiles of sea surface temperatures in many tropical oceans.

In particular, the phenomenon of coral bleaching, observed with increasing frequency during modern times, is a change in the structure of the coral produced by warmer than normal ocean temperatures. And if these temperatures remain high the corals can actually die. The Great Barrier Reef of Australia is one of the world’s most important stands of coral and is particularly vulnerable to coral bleaching. It is being closely monitored to detect any long-term damage from rising sea temperatures across the area.

Ocean and Lake Deposits

A great deal of climate information has also been locked away in the sediments that lie at the bottom of lakes and oceans – information that is proving to be of immense value in reconstructing past climates.

Of great assistance here have been the remains of tiny shell like creatures called foraminifera – normally less than 1 mm in size and difficult to see with the naked eye. Foraminifera live in the upper levels of oceans, normally near the surface, and after dying sink to the bottom in a continuous soft “rain”. Over thousands of years this forms a sediment which ultimately becomes fossilised.

Foraminifera evolved into many thousands of varieties that were and are temperature sensitive, and as scientists learned to interpret the countless layers of sediment they produced, many going back millions of years, a long term picture of changing ocean temperatures was assembled.

Interestingly some of the older freshwater lakes around the world also carry information from past climates in the form of pollen deposits that can sometimes be retrieved from the lakebeds. These can be dated and used to identify what sort of plant species were about in ancient times, and from these plant types, the prevailing climate deduced.

Tree Rings

Just as living coral formations contain a great deal of climate information within their structure, so do their land based equivalent – trees – that develop growth patterns that are also very dependant on the climate of the time.

If we look at the cross section of the trunk a tree that has been sawed through we notice it is made up of a number of concentric rings – the older the tree the larger the number. Each of these rings represents a year of growth and the width of each provides a valuable clue as to the climate of the time.

Series of well developed tree rings from an old tree. Each ring represents a year of growth (Image from Wikipedia Commons - click to enlarge)

A broad ring points to a year in which growth was rapid – a time of good rainfall and a suitable range of temperatures. During a season when the tree was stressed, the ring is narrower, representing a year in which the tree grew more slowly due to less suitable rainfall and temperature conditions.

The study of past climate through the analysis of tree ring growth is called dendrochronology and has proved of considerable value in the reconstruction of past climates, particularly when the trees in question are very old. This is certainly the case with the Bristlecone pines of Colorado and California that in some cases produce living specimens more than 4000 years old.

In addition, dendrochronological studies are not confined to living specimens – fossilised tree remains containing ring information that can be dated and analysed in the same way to produce information about far more ancient climates.

Temperature Timeline

From this mountain of evidence climatologists have been able to build up a rough picture of past climates, and the temperature trends have been shown to be highly variable over long periods of time. Before 600 million years ago (mya) little is known because of the lack of reliable proxy evidence available.

The approximate temperature trends of our past climate. The increased temperature variability in more recent times may be due to the increased amount of proxy data available.(mya means "million years ago"). Click on the image to enlarge.

The causes of these temperature variations is the subject of a great deal of research.